Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X
Vol. 5(6), 28-31, June (2015)
Res. J. Chem. Sci.
The Rapid Iodination of Salicylic Acid in Aqueous Medium by Iodine
Monochloride using Hydrodynamic Voltammetry
Ghorpade V.S., Borkar V.T., Bhosale P.R. and Dangat V.T.*
Post-Graduate Dept. of Chemistry, Nowrosjee Wadia College, Pune 411001, INDIA
Available online at: www.isca.in, www.isca.me
Received 20th May 2015, revised 6th June 2015, accepted 16th June 2015
Abstract
The rapid kinetics of iodination of salicylic acid by iodine monochloride at 7 pH has been studied at five different
temperatures. The reaction is very fast and of the second order, having half-life 300 seconds and rate constant 6.81 M-1s-1 at
30.20C. The reaction is fast and hydrodynamic voltammetry, a special technique to follow the rapid iodination kinetics is
employed. he energy of activation, entropy change in the reaction and the frequency factor in the reaction are evaluated. The
kinetic and related thermodynamic data obtained are used to comment on the reactivity of the substrate.
Keywords: Iodine monochloride, hydrodynamic voltammetry, salicylic acid, fast reaction.
Introduction
cell used in this hydrodynamic voltammetry technique.
Iodine monochloride is an interhalogen compound with the
formula ICl1. The molecular mass of iodine monochloride is
162.35 g/mol and density is 3.10 gcm-3. It is soluble in CS2,
acetic acid, pyridine, alcohol, ether and water.
ICl + 2e⁻ → I-+ Cl-
ICl is polar and is a potential source of I+. Iodinations of
aromatic substrates in aqueous medium yield iodo derivatives
that have significance in pharmacodynamics , as moieties of
drugs having antiseptic, disinfectant, antiviral and anti-bacterial
properties2.
The rapidity of halogenations of aromatic substrates in aqueous
medium necessitates the use of special techniques to measure
the reaction rates3-5. These include temperature jump, stopped
flow and pulse radiolysis techniques.
We have herein adopted a relatively simple yet efficient
technique, Hydrodynamic Voltammetry, to monitor the rate of
the fast iodination of salicylic acid by iodine monochloride in
aqueous solution.
Iodine monochloride being the lone species in the reaction that
is electroactive at a microelectrode its decay as the reaction
proceeds is monitored by determining the nanocurrent at a
platinum microelectrode that rotates at 1000 rpm. The following
are the half-cell reactions at the two electrodes in the galvanic
HOOC
HO
+ ICl ( aq. )
Salicylic acid
At the positive electrode, RPE
2Hg + 2Cl⁻ →Hg2Cl2 + 2e⁻ At the negative electrode, SCE
___________________________________________________
ICl+ 2Hg + 2Cl⁻ → Hg2Cl2+ I+ ClOverall reaction
The use of hundredfold molar KNO3 ensures linear
proportionality of the nanocurrent at the RPE generated by
iodine monochloride.
Preparation of Solutions: Iodine Monochloride: A stock
solution of iodine monochloride is prepared in double distilled
water. The strength of this solution is determined
iodometrically.
Salicylic Acid: The required weight of A.R. grade of salicylic
acid is used to prepare the stock solutions in water.
Buffer Solutions: Na2HPO4 and NaH2PO4 each of 0.4 molarity
are prepared as buffer solutions.
Potassium Nitrate: A.R. grade potassium nitrate is used to
prepare a stock solution of which is used as the supporting
electrolyte.
HOOC
HO
I +
H
( aq.)
+ Cl ( aq.)
5-Iodo Salicylic acid
Scheme-1
The reaction between Salicylic acid and ICl
International Science Congress Association
28
Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606X
Vol. 5(6), 28-31, June (2015)
Res. J. Chem. Sci.
Material and Methods
Construction of the RPE: The rotating platinum electrode
(RPE) consists of a 0.5 mm diameter platinum wire fused to one
end of inverted ‘T’ shaped glass tube having 6 mm diameter,
such that a centimeter of the platinum wire protrudes out6,7. A
pulley and a pair of ball-bearing are mounted on this glass tube,
having the total length of 32 cm. The ball bearings are fixed to a
stand. A pulley is connected to a synchronous motor. The radius
of the pulley is so adjusted that the electrode rotates at a speed
of 600 rpm. Some mercury kept inside the glass tube and a
silver wire is inserted for electrical contact. The lower 4cm
portion of the glass tube effects a stirring action in the solution
when the electrode is rotated. A constant potential of + 0.1V
versus the saturated calomel electrode (SCE) is applied at the
RPE, using a potentiometer.
A galvanometer with the sensitivity of 0.10 nA cm-1 provided
with a lamp and scale arrangement is employed to register the
nanocurrent due to iodine monochloride. The current passing
through the galvanometer is controlled by employing a shunt.
Calibration of Readings: The two electrodes are dipped in 50.0
cm3 of 5×10-2M KNO3 which is the supporting electrolyte. +
0.1V versus the reference electrode is applied at the rotating
platinum cathode. The galvanometer light spot is adjusted to
zero deflection on the scale. KNO3 solution is then replaced by
5×10-4 M iodine monochloride solution containing 5×10-2 M
potassium nitrate. The nanocurrent due to iodine monochloride
in the range 1×10-3 M to 5×10-4 M is recorded. A typical
calibration plot is depicted in figure1.
The procedure is repeated at five different temperatures using a
thermostat.
Kinetic Measurements: 25 cm3 of 1×10-3M salicylic acid
containing 0.1M KNO3 and 1×10-3 M iodine monochloride
containing 25 cm3 of 0.1 M potassium nitrate containing
Na2HPO4 and NaH2PO4 are maintained in a thermostat in
separate flasks. While mixing the contents of the two flasks in
the reaction vessel containing the electrodes, a stop-clock is
started and the attenuating nanocurrent due to ICl is monitored.
The above procedure of calibration and kinetic measurement is
repeated for checking the reproducibility of the galvanometer
measurements, and these are found to be within the limits of 0.2
cm.
Results and Discussion
From the deflections noted during the kinetic study, the
unconsumed iodine monochloride, is determined using figure-1.
A plot of [ICl]-1 i.e. 1/(a-x) versus ‘t’ is a straight line
ascertaining the order of the reaction to be two.
The gradient of this plot gives the specific reaction rate ‘k’.
Figure-1
Calibration Plots at various temperatures
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Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606X
Vol. 5(6), 28-31, June (2015)
Res. J. Chem. Sci.
Figure-2
Plot of reciprocal of concentration of unconsumed ICl versus time
Figure-3
Arrhenius Plot for the iodination of salicylic acid by ICl
International Science Congress Association
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Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606X
Vol. 5(6), 28-31, June (2015)
Res. J. Chem. Sci.
Cl
HOOC
HOOC
I Cl
HO
:
HO
HOOC
H
HO
I
I +
HCl (
aq.
)
Scheme-2
Mechanism of iodination of Salicylic acid by ICl
These studies are repeated in the temperature range 10-300C,
from which the energy of activation, ‘Ea’ for the reaction is
evaluated. Further, the frequency factor, A and the entropy
change ∆S are also calculated.
Table-1
Calibration of the diffusion current of iodine monochloride
at various temperatures for iodination of salicylic acid
( ± 0.2 nA error)
Diffusion Current/nA
-4
[ICl]/10 M
1
2
3
4
5
10.1°C
2.1
4.2
6.4
8.5
10.4
15.4°C
2.4
4.8
7.2
9.8
12.1
20.1°C
2.8
5.7
8.4
11.2
14.2
25.2°C
3.3
6.5
9.8
13.2
16.2
References
1.
30.2°C
3.6
7.2
10.8
14.4
18.0
Table-2
Kinetics of iodination of salicylic acid by using iodine
monochloride at 10.10C (±0.2na error)
Diffusion
[ICl]-1/103
Time/s
[ICl]/10-4 M
Current/nA
M-1
0
10.4
5.0
2.0
50
10.0
4.8
2.1
100
9.3
4.5
2.2
150
8.9
4.3
2.3
200
8.7
4.2
2.4
250
8.3
4.0
2.5
300
7.7
3.7
2.7
350
7.5
3.6
2.8
400
7.0
3.4
2.9
Table-3
Variation of rate constant of iodination of salicylic acid by
iodine (±0.2 error)
Temp/K
[T]-1/10-3 K-1
k/M-1s-1
log k
283.1
3.53
1.70
0.2304
288.4
3.46
2.50
0.3979
293.1
3.41
3.41
0.5321
298.2
3.35
5.10
0.7075
303.2
3.29
6.81
0.8331
Conclusion
The rate of electrophilic aromatic substitution reactions depends
on the reactivitity of the electrophile, and steric considerations.
International Science Congress Association
The substrate under study is acidic and a weak nucleophile8.
The electrophile is moderately strong and the bulkiness of the
incoming iodo group offers moderate steric hindrance. These
factors in unison slow down the iodination rate in comparison
with bromination of salicylic acid by bromine in aqueous
medium9. These facts have been quantitatively justified in the
present study.
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